What is the greatest common factor of $21$ and $30$ ? Another way to say this is: $\operatorname{gcf}(21, 30) = {?}$
The greatest common factor is the largest number that is a factor of both $21$ and $30$ The factors of $21$ are $1$ $3$ $7$ , and $21$ The factors of $30$ are $1$ $2$ $3$ $5$ $6$ $10$ $15$ , and $30$ Thus, the greatest common factor of $21$ and $30$ is $3$. $\operatorname{gcf}(21, 30) = 3$